Composite molded boards of fibrous or woody material bound together with a small amount of thermosetting resinous material have been known for thirty years or more. See for example U.S. Pat. No. 2,773,790; 2,835,622 and other patents of James d'A. Clark. These structural boards are made by dry process molding under heat and pressure and are referred to as waferboards, chipboards and similar terms. As used herein, these terms will be understood to include all types of dry process boards wherein the woody or fibrous elements are substantially dry (less than 25% moisture), which may or may not be coated with 1-2% wax (used to enhance the adherence of the resin particles to the wood), and wherein the thermosetting resinous binder material is introduced in a substantially dry powder form. This is in contrast to wet or moist processes for making particle board or the like in which the woody or fibrous elements are laid down in a slurry or the resinous binder is in a solution or other liquid form.
Although waferboard has only about 80% the strength of plywood, and must therefore be made thicker to achieve equal strength, it is much cheaper to make than plywood and has gained wide acceptance in the last five to ten years. However, due to the heavy competition among various types of molded composition boards, the economics of production of waferboards are quite critical.
Waferboard construction panels are manufactured by a dry process in which wood wafers are bound together with a thermosetting resin under high temperature and pressure. The resin is generally a phenol formaldehyde, although other thermosetting resins such as urea formaldehydes and melamine formaldehydes can be used. Prior to the application of heat and pressure in the molding step, the resin is in a dry powder form and an intermediate polymeric stage. For example, a B stage resin (thermoplastic) powder is mixed with the wood wafers. Upon application of heat and pressure, the resin melts to coat the wafers and is then cured to the C stage.
In the process of making waferboards, it is desirable to have resin particles which are fine enough to give good distribution when coating wafers, but not so fine that dusting becomes a health and housekeeping problem. Therefore, the waferboard industry aims for a resin powder which has predominantly particles between 10 and 74 microns.
Although the resin comprises only about 1 to 4, and preferably about 2 - 2.5 weight percent of the finished board, the resin is expensive, and even a 1/4 percent increase or decrease in the resin required can make a large difference in the economics of waferboard production. It is therefore important that there be good dispersion of the resin on the wood fiber elements (e.g., wafers). To this end large particles must be minimized or eliminated and small particles, but not fines, must predominate.
Powders of phenol formaldehyde resins are typically produced by spray drying an aqueous solution of the resin (see for example U.S. Pat. No. 4,098,770). However, commercially available atomization systems used in spray drying phenol formaldehydes do not achieve the desired particle size distribution. Thus, if drying conditions are adjusted to minimize the fine particles (less than 10 microns), a large percentage of the particles are over 74 microns in diameter. Conversely, if drying conditions are adjusted to minimize the oversize particles (greater than 74 microns), an appreciable amount of fines is formed.
Although it is possible to eliminate fines and oversize particles by sieving and various other particle classification methods, such procedures add expensive additional steps to the overall production of the waferboards. It would therefore be desirable to be able to obtain greater control of the particle size distribution during the spray drying of the resin solution so that the spray dryer output yields a larger percentage of particles in the 10 to 74 micron range.
Apparatus and methods are known which can produce macrospherical particles of antiperspirant materials, such as aluminum, zirconium and magnesium compounds and complexes, with particle size distributions predominantly in the 10 to 74 micron range. Such methods and apparatus are described in U.S. Pat. Nos. 4,089,120; 4,147,766 and 4,430,155, all assigned to the same assignee as the present invention. The process described in U.S. Pat. No. 4,089,120 and 4,147,766 comprises providing a solution containing the materials from which the particles are made, diffusing the solution through small pores by centrifugal force such that the resulting solution droplets have a diameter greater than the pore diameter, and drying said solution droplets in a stream of heated air. The apparatus described in these patents comprises a centrifugal atomizer having a porous sintered metal filter ring which is rotated inside a spray drying chamber.
The process of U.S. Pat. No. 4,430,155 comprises providing a solution containing the materials from which the particles are made, dispersing the solution from a central source outwardly along a plurality of radially disposed bristles by centrifugal force to form discreet liquid droplets, and drying the droplets in a stream of heated air to form the particles after the droplets leave the free ends of the bristles. The apparatus of that patent comprises a centrifugal atomizer having a plurality of radially outwardly extending bristles extending from fluid outlets from a generally centrally located solution source, the atomizer being rotated in a spray drying chamber
Although these three patents prophetically mention the possibility of using those methods and apparatus for the production of macrospherical particles having many areas of use, including pigments, resins, catalysts, etc., the preferred use of such methods and apparatus was to produce particles of antiperspirant material. There was no specific suggestion in those patents of using the methods and apparatus for spray drying any specific resins, and to our knowledge, until the present invention none of these methods or apparatus had been used to spray dry any resins. Moreover, in view of the high feed rates which are required in the spray drying of phenolic and other thermosetting resins for the waferboard industry, it was not expected that the porous metal atomizers and methods of U.S. Pat. No. 4,089,120 and 4,147,766 would be suitable for spray drying resins of this type.